Thermistors and methods of making same



Jan. 30, 1962 F. H. FASTENAU ETAL 4 THERMISTORS AND METHODS OF MAKING SAME Filed Dec. 22, 1955 1. '4. 5W /fl jg ffE/fitzyer INVENTOR) ATTORNEYS.

United. States. Patent- O 3,019,404 THERMISTORS AND METHODS OF MAKING SAME Frederick H. Fastenau, Huntington Station, and Seymour C. Spielberger, Roslyn Heights, N.Y., assignors to Bulova Research and Development Laboratories, Inc, Woodside, N.Y., a corporation of New York Filed Dec. 22, 1955, Ser. No. 554,678 4 Claims. (Cl. 338-28) This invention relates generally to thermally-sensitive Elements so dimensioned are highly sensitive to minute amounts of radiant heat.

One form of thermistor assembly commonly, used for the detection of infra-red rays is made up of four flake elements of thermistor composition, the flakes being cemented to blocks and arranged in the configuration of a cross. Heretofore such devices have usually been made by milling and compounding triple oxide thermistor material with a plastic binder and spreading the paste thus formed on an optical flat. After permitting the paste to dry, the resultant film is then stripped carefully from the flat to yield a thin plastic sheet. This sheet is precisely cut into individual flakes and loaded into a sintering furnace. Following the sintering operation, the flake takes the form of an extremely thin and fragile element having athickness in the order of 10 microns. Further processing involves the painting of metallic contact areas on the individual flakes and the attachment of leads thereto. The flakes are cemented onto a supporting block to complete the thermistor structure.

The above-described steps in the making of the conventional thermistors are of necessity slow and tedious, and call for a high degree of skill as well as rigid controls. Because of their fragility the flakes can safely be handled only by a vacuum pipette. One of the problems encountered is a tendency for the flakes, particularly the thin ones, to curl and warp during the sintering process. This curling or warpage may arise from uneven shrinkage of the flake in the course of sintering. Such imperfect flakes are unacceptable and must be re jected. Moreover, a large number of flakes are broken or damaged because of their extreme fragility.

It has heretofore been the practice to cement or otherwise secure'the small, thin sintered flake on a relatively large slab or backing. This backing serves not only to support the flake but also acts as a thermal sink which governs the heat response characteristic of the thermistor. When radiant energy impinges upon the flake, it is rapidly heated, but when the radiant energy is removed, the backing carries the heat away to effect cooling thereof.

The speed of heat dissipation from the thermistor flake depends on the material employed for the backing. In order that the thermistor element shall heat rapidly when energy falls on it, its heat capacity should be slight, hence its dimensions should be small. To make the cooling rate high, the thermal conductivity of the backing should be high. It is also important that the thermal diffusivity of the backing be large as compared to that of the flake material whereby the heat capacity of the combination is practically that of the flake alone. Thermal diffusivity may be defined as therice.

mal conductivity divided by heat capacity per centimeter cube.

To produce a bolometer with a short time constant, it is necessary to keep the heat capacity low and make the dissipation constant high. This dissipation constant is determined by the watts that must be supplied to the thermistor to maintain its temperature 1 C. above its surroundings. It is recognized that good thermal contact between the thermistor element or flake and its backing is essential to alarge dissipation constant. Inasmuch as the conventional thermistor is secured to its backing by a cement bond, a good thermal contact therebetween is not uniformly attained in practice.

Accordingly, it is the principal object of this invention to provide an improved thin film thermistor structure of high sensitivity and to facilitate the manufacture of such thermistors.

More particularly, an object of the invention is to provide a simpler and more eflicient process for making 'thermistors, eliminating a number of steps previously entailed. A method in accordance with the invention obviates the extraordinary care hitherto required in thermistor manufacture and makes possible a substantial economy in production time and costs. Asignificant advantage of the invention resides in the fact that the thermistor film, which is formed on a backing and never stripped therefrom in the course of manufacture, is not subject to curling or Warpage, and there is no breakage experienced in the manufacturing process.

Also an object of the invention is to provide athermistorwherein a film of thermistor material is sintered on a ceramic backing and adheres firmly thereto without the intermediary of a cement layer or other adhesive means. A thermistor so formed is characterized by intimate thermal contact between the thermistor element and its backing.

Briefly stated, a thermistor in accordance with the invention is fabricated by spreading a paste of a thermistor material mixture in a cavity formed in a ceramic block, the block preferably being of sintered aluminum oxide. The paste is permitted to dry to form a thin film on the surface of the block and the combination is then fired at a temperature sufficient to sinter the film without adi-other objects and further features thereof, reference is had to the following detailed description to be read in conjunction with the accompanying drawing, wherein like components in the various views are identified by like reference numerals. In the drawing:

FIGURE 1 is a perspective view of ture in accordance with the invention.

FIGURE 2 isa transverse section taken through the plane of line 2-2 in FIG. 1.

FIGURE 3 illustrates in plan view an arrangement of thermistor elements in a bolometer.

FIGURE 4 is the electrical circuit of the bolometer.

Referring now to the drawings, a thermistor structure in accordance with the invention is constituted by a ceramic slab or backing 10 supporting a thermistor element 11. In view of the smallness of the thermistor elements, it has been necessary to exaggerate some proportions in the drawing in order adequately to illustrate the invention.

The backing 10 has a generally rectangular shape, one end of the backing being tapered to a point. A narrow groove 12 cut into the upper surface extends longitudinally along the center of the backing to form a shala thermistorstruc- 3 low cavity or channel for accommodating the thermistor element. Metallic coatings 13 and 14 are applied to either end of the element to provide contact areas. Leads 15 and 16 are secured to the contact areas by any suitable means such as a metallic paste that may be heat cured to form a bond.

The backing is made of an aluminum oxide ceramic block. This block is of very pure and highly compacted material which is sintered at high temperature, in the order of 1950 degrees centigrade. The thermistor material employed may be selected from various substances having relatively high temperature coefiicient of resistance. For example, one or more oxides of manganese, nickel, cobalt, copper, iron or zinc may be employed in finely divided granulated form. Good results have been obtained with a thermistor material comprising the combined oxides of manganese, nickel and cobalt.

To make a paste, a quantity of the thermistor material is thoroughly mixed as in a ball mill with a temporary binder such as polymerized methacrylates, polyvinylbutyral or cellulose acetate butyrate and a volatile solvent.

In forming the thermistor element, a very fine film of thermistor material paste is spread out in the groove 12 on the block, so that on drying its thickness will be in the order of tens of microns. The film is then dried in a dust-free atmosphere, this being accomplished by passing air over it at room temperature to evaporate the volatile binder solvent.

The film and its aluminum oxide backing are then fired in an oven at the normal sintering temperature of the thermistor material. This temperature generally is in the range of 1100 to 1450 degrees C., which is substantially below the temperature of the previously sintered backing. The sintering operation results in an intimate bond between the sintered thermistor film and the ceramic backing, the film being free of surface cracks or other defects. The individual thermistor film may now have the metallic coating applied to the end portions thereof to form the contact areas and the leads may be attached thereto.

Since the film is sintered on its backing to form the thermistor flake, which flake in the course of sintering bonds itself to the backing, it is not necessary to strip the flake, as was the case in the prior art. The thermistor thus formed is supported during sintering and there is no tendency of the thermistor flake to curl or warp. Nor is then any need to exercise exceptional care in fabrication.

To form the bolometer, four thermistor elements, as shown in FIG. 3, are symmetrically arranged at right angles to each other with their pointed ends together to define a cross-shaped structure. To operate the four thermistor elements as a bolometer, they are serially connected, as shown in FIG. 4, to form an electrical bridge circuit whose balance is determined by the relative resistance values of the thermistor elements.

While there has been shown what is considered to be a preferred embodiment of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims to cover all such changes and modifications as fall within the true scope of the invention.

What is claimed is:

1. A thermistor structure comprising a ceramic backing constituted by a densely compacted aluminum oxide block, and a flake of thermally sensitive resistance material intimately bonded to the surface of said backing by sintering, said bond between said backing being free of an intermediate adhesive.

2. A thermistor structure comprising a ceramic backing constituted by a densely compacted aluminum oxide block, and a flake of thermally sensitive resistance material intimately bonded to the surface of said backing by sintering, said bond between said backing being free of an intermediate adhesive, said flake being disposed Within a longitudinal groove in said block, said block being pointed at one end, whereby four of said thermistor elements may be symmetrically arranged with their pointed ends together to constitute a cross-shaped bolometer arrangement, metallized coatings on each end of said flake to provide electrodes, and leads secured to said electrodes.

3. The method of manufacturing a thermistor comprising the steps of spreading a paste of finely divided triple oxide thermistor material in a cavity formed on a sintered aluminum oxide block to produce a thin film thereon in the order of 10 microns thickness, said block having sintering temperature above that of said material, and firing said film on said block in a temperature range of 1100 to 1450 degrees centigrade to sinter said film and to efiect an intimate bond between said film and said block.

4. The method set forth in claim 3 wherein said thermistor material comprises the combined oxides of manganese, nickel and cobalt mixed in a temporary binder.

References Cited in the file of this patent UNITED STATES PATENTS 1,747,664 Droitcour Feb. 18, 1930 1,767,715 Stoekle June 24, 1930 1,847,653 Jones Mar. 1, 1932 2,067,604 Godsey Jan. 12, 1937 2,414,792 Becker Jan. 28, 1947 2,414,793 Becker Jan. 28, 1947 ,423,476 Billings July 8, 1947 2,434,560 Gunter Jan. 13, 1948 2,633,521 Becher et a1. Mar. 31, 1953 2,765,385 Thomsen Oct. 2, 1954 2,768,265 Jenness Oct. 23, 1956 2,793,275 Breckenridge et al May 21, 1957 FOREIGN PATENTS 273,039 Great Britain June 30, 1927 OTHER REFERENCES Infrared Photoconductors, by R. A. Smith, Philosophical Magazine Supplement, vol. 2, No. 7, July 1953, pp. 321-369, only page 334 relied on.

Electrical Engineering, Transactions Section, November 1946, pp. 711-725. 

1. A THERMISTOR STRUCTURE COMPRISING A CERAMIC BACKING CONSTITUTED BY A DENSELY COMPACTED ALUMINUM OXIDE BLOCK, AND A FLAKE OF THERMALLY SENSITIVE RESISTANCE MATERIAL INTIMATELY BONDED TO THE SURFACE OF SAID BACKING BY SINTERING, SAID BOND BETWEEN SAID BACKING BEING FREE OF AN INTERMEDIATE ADHESIVE. 